Dimensional accuracy refers to the degree to which the size of the workpiece after processing conforms to the size requirements of the drawing. The extent to which the two do not match is usually measured by the size of the error. The errors include machining errors, installation errors and positioning errors. Among them, the latter two errors are related to the positioning and installation of the workpiece and the tool, and have nothing to do with the processing itself. In order to improve the machining accuracy and reduce the machining error, we must first choose a high-precision machine tool to ensure the installation and positioning accuracy of the workpiece and the tool, and then mainly related to the processing technology of water wheel rotary transfer machine.

Each component of the process system, including machine tool, tool, fixture manufacturing error, installation error, wear in use directly affect the machining accuracy of the workpiece. In other words, the process system will produce a variety of errors during the machining process, thereby changing the mutual position relationship between the tool and the workpiece in the cutting process and affecting the machining accuracy of the part. water wheel rotary transfer machine processing, professional quality assurance, these errors are related to the structural state of the process system itself and the cutting process, the main factors of processing errors are:

1, processing principle error

Machining principle error is due to the use of approximate machining motion or approximate tool profile caused by the error, because there is an error in the machining principle, so called machining principle error. As long as the principle error is within the allowable range, this processing method is still feasible.

2. Geometric error of the machine tool

The manufacturing error, installation error and wear of the machine tool directly affect the machining accuracy of the workpiece. Among them, the main errors are the rotary movement of the machine tool spindle, the linear movement of the machine tool guide rail and the machine tool transmission chain.

3, the manufacturing error and elastic deformation of the tool

Many of us have such an experience, that is, after cutting a few millimeters of depth in the previous knife, we found that we are still a few or a dozen wires short of the desired size, and then when we plan to cut the next knife, we found that we cut a lot more, and the size may be out of line. Have we seriously analyzed the reasons for such a situation? Some people say that this may be because the gap of the machine tool is relatively large, and in the same feed direction is not affected by the gap, the real reason is elastic deformation and elastic recovery.

Elastic deformation is manifested in the deformation of tools, machine tool screw pairs, tool holders, machining parts themselves and other objects, so that the tool is backward relative to the workpiece, and the deformation recovery will be overcut when the resistance is reduced, so that the workpiece is scrapped. The ultimate cause of deformation is that the strength of these objects is insufficient and the cutting force is too large.

Elastic deformation will directly affect the dimensional accuracy of the parts processing, and sometimes affect the geometric accuracy (such as the taper is easy to produce when the parts are deformed, because the larger the deformation distance from the chuck location), the strength of the tool is insufficient, we can try to improve, sometimes the strength of the machine tool and the parts themselves, we can not choose or change, so we can only start from the reduction of cutting force, To try to overcome elastic deformation, the smaller the cutting depth, the sharper the tool, the lower the hardness of the workpiece material, and the reduction of the cutting speed will reduce the actual cutting resistance and reduce the elastic deformation.

So in order to ensure the size of the workpiece with field accuracy, we often separate finishing, semi-finishing and roughing, that is to say, the different processes of large elastic deformation and small elastic deformation are separated (roughing the pursuit of efficiency is basically not the pursuit of precision, the tool needs to be dull, focusing on strength, the cutting amount is very small, the pursuit of precision, the tool focuses on sharp, Reduce the cutting resistance), in the knife test cutting, according to the actual cutting depth of different processes, to ensure that the resistance and elastic deformation amplitude of the test cutting and the actual processing are roughly equivalent, to ensure that the CNC machine tool coordinate system is established accurately, to ensure that the ordinary machine tool feed is accurate; Then, as far as possible, sharp tools are used in finishing to minimize the sensitive cutting resistance and reduce deformation.

The manufacturing error, installation error and wear of the tool affect the machining accuracy of the workpiece. In the cutting process of the tool, the cutting edge, the tool face and the workpiece, the chip produce strong friction, so that the tool wear. When the tool wear reaches a certain value, the workpiece surface roughness value increases, the chip color and shape change, and accompanied by vibration. Tool wear will directly affect cutting productivity, machining quality and cost.

4, fixture error

The fixture errors include positioning errors, clamping errors, fixture installation errors and tool alignment errors, which are mainly related to the manufacturing and assembly accuracy of the fixture.

4.1. Reference non-coincidence error

When the positioning datum and the process datum do not coincide, the processing error is called the reference non-coincidence error, the size of which is equal to the size tolerance between the positioning datum and the process datum.

4.2. Reference displacement error

When the workpiece is positioned in the fixture, due to the influence of the manufacturing tolerance and minimum fit gap of the workpiece positioning base surface and the limit base surface of the positioning element on the fixture, the positioning reference and the limit reference can not coincide, so that the position of each workpiece is inconsistent, causing an error to the processing size, which is called the reference displacement error.

5, the effect of speed on processing

Under normal circumstances, we all know that the higher the speed, the higher the efficiency of cutting, efficiency is profit, so we have to run as high a speed as possible to cut under conditions. However, the speed and diameter of the workpiece are determined by the cutting line speed, which is restricted by the workpiece hardness, strength, plasticity, carbon content, the amount of hard cutting alloy and the hardness and geometric properties of the tool, so the speed should be selected as high as possible under the online speed limit. In addition, the speed level selection should be determined according to the tool of different materials, such as high-speed steel processing steel parts, the roughness is better when the speed is low, and the cemented carbide tool is better when the speed is high. Moreover, when processing slender shafts or thin-walled parts, attention should be paid to adjusting the speed to avoid the resonance area of the parts to prevent vibration patterns from affecting the surface roughness.

6, the impact of cutting elements on surface roughness

We know that when the workpiece material is hard, the surface roughness of the workpiece is better after processing, and when the plasticity and ductility of the workpiece material are higher (such as copper, aluminum), the more sharp the tool is needed to process the better surface roughness, gray cast iron processing relative to steel processing, because the composition is complex, containing high degree of impurities, you need higher tool hardness. Some alloy materials with high ductility and strength need sharp but strong tools, so it is more difficult to process (such as stainless steel, nickel-based heat-resistant alloys, titanium alloys, etc.).

In addition to the requirements of the material on the tool, the cutting elements will also have an impact on the surface roughness, when the finishing cutting depth is too small, or even thicker than the tool edge, the blade can not achieve normal cutting, so the extrusion, there will be a very poor surface roughness. When the cutting depth is too large, and even the tool bends, the workpiece material is torn down, so there will be a lot of filamentous iron filings left on the workpiece and more obvious lines. The influence of cutting speed on the surface roughness of the workpiece is also quite obvious, when the cutting speed is accelerated or the tool pair deflection Angle is inappropriate, the cutting grain height will be increased, which will make the surface roughness worse.

When the tool is not very sharp, the cutting depth is too small, even less than the thickness of the blade, it is not normal cutting, it can only belong to "scraping" or "grinding", the surface roughness of the workpiece will decrease, and the surface of the workpiece appears fine white wire, as if enveloped in a layer of white fog, so pay attention to control.

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